1. Field of the Invention
The present invention relates to a head positioning control system for a magnetic disk device and a method for the same.
2. Related Art
In a head positioning control system of a magnetic disk device, control for making the head follow the same track is called following (or tracking) control. A basic structure of a typical following control system includes a feedback controller for calculating control input from a detected position error signal and a resonance filter for preventing the control system from being unstable due to machine resonance excitation of head gimbal assemblies (e.g., an arm and a suspension). Conventional design methods for feedback controllers employ PID control (phase lead/delay compensation), LQG control, H∞ control, and so forth. In principle, however, many of such methods reduce to a linear feedback controller that combines an integral element for low-frequency compensation, phase lead compensation for securing stability margin, and a resonance stabilizing element having a notch filter shape (for example, see JP-A 2006-79670 (Kokai), JP-A 2006-179185 (Kokai), Hirata, Ryu, et al., “Head positioning control for a hard disk using H∞ control theory”, Transactions of the Society of Instrument and Control Engineers, Vol. 29, No. 1, pp. 71-77, (1998)).
Since making the capacity of a magnetic disk large requires making a track pitch be of high density, performance improvement of a following control system that governs the accuracy of head positioning is a critical issue.
The accuracy of head positioning of a following control system is determined by how a sensitivity function of the feedback control system is shaped. In design of a general control system, sensitivity for a frequency band in which disturbance (which is primarily low-frequency disturbance sufficiently lower than a crossover frequency (a frequency at which a sensitivity function intersects the 0 dB line)) is significant should be reduced. In a head positioning control system of a magnetic disk, however, DC, torque disturbance, low-order RRO disturbance or the like exist in lower frequency bands, flutter disturbance exists around the crossover frequency, and mechanical resonance disturbance due to windage exists in higher frequency bands, meaning that disturbance is distributed across a wide band. Thus, with a conventional linear feedback controller, tradeoff of control performance caused by waterbed phenomenon of the sensitivity function is inevitable and limit of control performance is fixed.